Within the past few years light-emitting diode (LED) technology has advanced to a point where the efficiency of light generated by an LED array matches or even exceeds the efficiency of incandescent lamps. In many lighting applications, red, green and blue LEDs are employed to generate a conventional white light. By properly mixing the light generated by each group of the red, green and blue LEDs it is possible to control the colour temperature of the white light generated by the LEDs. Theoretically, the colour temperature of a light source is defined in terms of the temperature of an ideal purely thermal light source also known as a Planck or black body radiator whose emitted light spectrum has the same chromaticity as that of the light source. The colour temperature is typically measured in Kelvin because a black body at that temperature emits a light spectrum of that specific chromaticity. Even when the chromaticity or the colour temperature is the same, the light source and the black body radiator may have different spectral density distributions which may lead to differences in the observable colour rendering. A measure for this deviation can be defined in terms of a colour rendering index which defines how well colours are rendered by different light sources in comparison to a reference standard.
The term chromaticity is applied to identify the colour of the light source regardless of its brightness or luminance. Brightness or luminance is typically measured in candela/cm2. When the chromaticity of different light sources is equal, the colour of the light from each light source is most likely to appear the same to the eye of a human standard observer regardless of the lighting level. The chromaticity of a light source can be represented by chromaticity coordinates. An example of such coordinates is the CIE (Commission Internationale de l'Eclairage) 1931 chromaticity diagram, in which the colour of the emitted light is represented by x and y coordinates.
Having regard to LED based luminaries, the energy efficiency, overall system effectiveness, colour uniformity, colour rendering, and economic viability of a white light generating luminaire can greatly depend on the specific characteristics of the kinds of LEDs which are employed as light sources in the colour mixing process preformed by the luminaire.
United States Patent Application No. 2005/0030744 discloses a white light LED luminaire system comprising two white light LED sources of different correlated colour temperature. The system can generate variable colour temperature white light when mixed with an additional colour, for example, amber. This invention however, is based on a relatively inefficient utilization of two different colour temperature white light LED sources because each white light source must be dimmed considerably under almost all operational conditions in order to achieve a desired intermediate correlated colour temperature white light emission. Consequently, the luminaire of this system can require approximately twice as many white light LED sources to create a desired colour temperature white light impression than may otherwise be necessary.
Thus, there is a need for a luminaire system that can effectively combine a reduced number of light sources that can maintain a specified, and in particular, an adjustable correlated colour temperature at a desired brightness for required operating conditions.
This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.